A light emitting diode is a device directly excited by a current to emit light. At present, light emitting diodes include a Quantum dot light-emitting diode (QLED) and an Organic Light-Emitting Diode (OLED).
FIG. 1 is a structural diagram of a conventional light emitting diode. As shown in FIG. 1, the light emitting diode includes an anode 10, a hole transport layer 20, a light emitting layer 30, an electron transport layer 40 and a cathode 50 which are sequentially disposed. A principle of light emission of the light emitting diode is: under an action of an electric field, holes are injected from the anode 10 into a valence band of the light emitting layer 30 and migrate toward the cathode 50, and electrons are injected from the cathode 50 into a conduction band of the light emitting layer 30 and migrate toward the anode 10. In a case in which the holes and the electrons are injected into an energy level of the valence band and an energy level of the conduction band of the light-emitting layer 30 respectively, an exciton (electron-hole pair) is formed under the Coulomb force, an electron in an exciton state undergoes a radiation transition, and energy is released in a photon form, thereby an electroluminescence is achieved.
However, when the light emitting diode is a QLED, a material of the light emitting layer 30 is a quantum dot light emitting material. Exemplarily, if a HOMO (highest occupied molecular orbital) energy level of a material of the hole transport layer 20 is 5.0˜6.0 eV, and the energy level of the valence band of the quantum dot is 6.0˜7.0 eV, there is a large hole injection barrier at the interface between the hole transport layer 20 and a quantum dot light emitting layer 30. Since the energy level of the conduction band of a material of the electron transport layer 40 such as ZnO nanoparticles is close to the energy level of the conduction band of the quantum dot, there is a problem that electrons injection and holes injection are unbalanced. A luminous intensity is proportional to the product of electrons concentration and holes concentration. In a case in which the electrons concentration and the holes concentration are same, the luminous intensity is maximum. In a case in which a difference between the electrons concentration and the holes concentration is larger, the luminous intensity is smaller, and excess carriers will cause Joule heating, thereby a lifetime of the device is shortened.
In a case in which the light emitting diode is an OLED, the material of the light emitting layer 30 is an organic electroluminescent material, and an injection capability of a carrier is proportional to a mobility of a material and a square of a voltage applied to the carrier, and the injection capability of the carrier is inversely proportional to a thicknesses of the electron transport layer 40 or to a thicknesses of the hole transmission layer 20. In an organic electroluminescent diode, the thicknesses of the electron transport layer 40 and the thickness of the hole transport layer 20 are both approximately several tens of nanometers, and are of the same order of magnitude. Furthermore, in the organic electroluminescent diode, when a voltage is applied to the anode 10 and the cathode 50, it may be approximately assumed that an electric field intensity falls uniformly on the electron transport layer 40 and the hole transport layer 20. However, if the mobility of the material of the electron transport layer 40 is lower than the mobility of the material of the hole transport layer 20, it will lead to an imbalance between an injection of the electrons and an injection of the holes, thereby affecting the luminous efficiency and the lifetime of the organic light-emitting diode.